1. Field of the Invention
The present invention relates in general to a cushioning apparatus for applying a uniform cushioning force or a blank-holding force to a blank to be processed on a press, and more particularly to such cushioning apparatus capable of changing the force applied to hold the blank during a pressing operation on the workpiece, a method of checking or determining an optimum condition relating to a hydraulic pressure of the cushioning apparatus for uniform distribution of the blank-holding force, and a device for determining or establishing such optimum condition.
2. Discussion of the Related Art
A press has a press slide with an upper die attached thereto, which is lowered toward a lower die to perform a pressing operation on a blank or workpiece between the upper and lower dies while the blank is held by and between the upper die and a pressure member such as a pressure ring. For holding the blank during a pressing cycle, there is known a cushioning apparatus which includes (a) a cushion platen biased in an upward direction with a predetermined biasing force by suitable force generating means, (b) a plurality of balancing hydraulic cylinders disposed on the cushion platen and having respective fluid chambers which communicate with each other, and (c) a plurality of cushion pins linked at their lower ends with the respective hydraulic cylinders and supporting at their upper ends the pressure member, so that a blank-holding force (cushioning force) corresponding to the biasing force generated by the force generating means is applied to the pressure member through the cushion platen, hydraulic cylinders and cushion pins. The balancing hydraulic cylinders function to assure uniform distribution of the blank-holding force on the cushion pins, that is, permit the blank-holding force to be transmitted to the blank such that substantially equal components of the force act on all the cushion pins.
An example of such cushioning apparatus is disclosed in laid-open Publication No. 1-60721 of unexamined Japanese Utility Model Application, in which the hydraulic pressure in the balancing hydraulic cylinders is determined depending upon a desired value of the blank-holding force and other parameters, according to a suitably formulated equation or by a test pressing operation, so that the pistons of all the hydraulic cylinders are positioned between their upper and lower stroke ends (not positioned at their stroke ends) under reaction forces received from the cushion pins during a pressing action on the blank, irrespective of length variations of the cushion pins, inclination of the cushion platen, and other undesirable factors of the apparatus. The cushioning apparatus arranged as described above ensures substantially even distribution of the holding force and the surface pressure over the entire area of the pressure member, which permits consistently high quality of the articles produced from the blanks.
Another example of a cushioning apparatus for a pressing machine is disclosed in laid-open Publication No. 61-190316, in which the force generating means for generating the blank-holding force to be applied to the pressure member has a combination of a cushioning pneumatic cylinder and a cushioning hydraulic cylinder whose pistons are integrally connected in series with each other. The cushioning hydraulic cylinder is connected to a pressure relief valve, through which the pressurized fluid is discharged from the hydraulic cylinder when the hydraulic pressure exceeds a controllable relief pressure. The blank-holding force generated by the force generating means is determined by the pressure of the compressed air supplied to the pneumatic cylinder and the relief pressure of the pressure relief valve. During a pressing operation, the relief pressure of the relief valve is controlled by a controller, to change the blank-holding force. This cushioning apparatus having such force generating means and controller is capable of effecting fine or intricate adjustment of the blank-holding force during a pressing operation, namely, during a downward movement of the press slide after the upper die contacts the blank to start an effective pressing stroke down to the lower stroke end. The fine adjustment of the blank-holding force permits reduction in the vibration of the blank upon abutting contact of the upper die with the pressure member, and is effective to prevent an excessive amount of initial movement of the blank between the pressure member and the upper die due to the vibration, thereby assuring comparatively improved surface quality of the article produced from the blank. Further, the present arrangement makes it possible to reduce the blank-holding force during a pressing action on the blank to thereby protect the blank from rupture, whereby the material grade of the blank may be lowered to reduce the material cost. Thus, the cushioning apparatus having the force generating means and the controller arranged as described above not only assures even distribution of the blank-holding force on the pressure member (blank), but also provides other advantages for enhanced results of the pressing operation on the blank.
However, the above cushioning apparatus having the cushioning pneumatic and hydraulic cylinders as the force generating means capable of controlling the blank-holding force requires two hydraulic circuits, one for the balancing hydraulic cylinders associated with the cushion pins, and the other for the cushioning hydraulic cylinder of the force generating means for generating the blank-holding force. Accordingly, the apparatus tends to be complicated and expensive. Although the cost of the force generating means and the controller is comparatively low if they are incorporated in a cushioning apparatus upon manufacture of the apparatus, the application of these force generating means and controller to the existing cushioning apparatus requires considerable modification of the apparatus, which increases the overall cost of the apparatus to such a high level that cannot justify the practical utilization of such force generating means and controller for the existing cushioning apparatus.
As indicated above, the even distribution of the blank-holding force on the pressure member and the blank requires the pistons of all the balancing hydraulic cylinders associated with the cushion pins to be positioned between their upper and lower stroke ends, that is, held in their neutral position during a pressing operation on the blank, irrespective of possible length variations of the cushion pins and other fluctuating factors. To this end, an optimum initial hydraulic pressure Po applied to the hydraulic cylinders prior to a pressing operation so as to establish the desired even distribution of the blank-holding force on the pressure member is determined so as to satisfy the following equation (1): EQU Xav=(Fs-N.multidot.As.multidot.Po)V/n.sup.2 .multidot.As.sup.2 .multidot.K(1)
where,
Xav: average operating stroke of the pistons of the hydraulic cylinders (cushion pins); PA1 As: pressure-receiving area of the piston of each hydraulic cylinder; PA1 K: volume modulus of elasticity of the working fluid; PA1 V: total fluid volume in the hydraulic cylinders and the hydraulic circuit connected thereto; PA1 Fs: blank-holding force; and PA1 n: number of the hydraulic cylinders (cushion pins). PA1 Fs: blank-holding force; PA1 Wr: weight of the pressure member; . PA1 Wp: average weight of the cushion pins; PA1 n: number of the cushion pins; and PA1 As: pressure-receiving area of the piston of hydraulic cylinders.
The average operating stroke Xav of the pistons of the balancing hydraulic cylinders is predetermined by experiments, for example, so as to enable all the cushion pins to abut at their upper ends on the pressure member while the pistons of the hydraulic cylinders are spaced away from their upper stroke ends by the cushion pins, but do not reach their lower stroke ends due to collision of the upper die with the pressure member through the blank during a pressing action on the blank, even if the cushion pins have different length dimensions and the cushion platen is inclined with respect to the horizontal plane. The total fluid volume V is a total volume of the working fluid in the fluid chambers of all the hydraulic cylinders when the pistons are at their upper ends, plus a volume of the fluid in the hydraulic circuit connected to the hydraulic cylinders.
The optimum initial hydraulic pressure Po determined so as to satisfy the above equation (1) basically permits the pistons of the hydraulic cylinders to be lowered from their upper stroke ends by an average distance equal to Xav, during a pressing action on the blank, so that the blank-holding force is substantially evenly distributed on the pressure members through the cushion pins. In this respect, it is noted however that the volume modulus of elasticity K of the fluid varies due to air introduced into the fluid or deterioration of the fluid. Therefore, the optimum initial hydraulic pressure Po calculated according to the above equation (1) is not necessarily accurate. To improve the accuracy of the optimum initial hydraulic pressure Po, it is known to adjust the value Po as obtained from the equation (1), during a test pressing operation, so that an in-process hydraulic pressure PX generated in the hydraulic cylinders during the pressing operation substantially coincides with an optimum value PXo which is obtained according to the following equation (2): EQU Fs+Wr+n.multidot.Wp=n.multidot.As.multidot.PXo (2)
where,
The values Xav, As, K, V and other values necessary to calculate the optimum initial hydraulic pressure Po prior to a pressing operation and the optimum in-process hydraulic pressure PXo during the pressing operation should be as accurate as possible. In this sense, these values should not be theoretical values but should be obtained by experiments performed on the individual pressing machines which have specific operating characteristics. These experiments to obtain the values used to calculate the optimum initial and in-process values Po and PXo are extremely cumbersome and time-consuming. Yet, the values obtained by the cumbersome experiments may include some errors, which lead to errors in the calculated optimum values Po and PXo, resulting in the failure to establish uniform distribution of the blank-holding force and the surface pressure on the pressure member, even if the actual initial and in-process pressure values P and PX are adjusted to the calculated optimum values Po and PXo. Thus, the article produced from the blank may be defective. Another proposed approach to improve the quality of the produced article is to compare the actual in-process hydraulic pressure PX with the optimum value PXo obtained as described above, to thereby monitor the distribution of the blank-holding force on the pressure member. However, the comparison of the values PX and PXo does not allow accurate checking of the uniform distribution of the blank-holding force.